The ever-increasing demand for wireless communication services, such as cellular mobile telephone (CMT), digital cellular network (DCN), and personal communication services (PCS), requires the operators of such systems to attempt to make maximum effective use of the available radio channels. To that end, the geographic territory covered by such systems is typically divided into a number of portions, called cells. The system operator splits up the allocated frequency channels among the cells, so that units operating in adjacent cells do not interfere with one another. Base stations are deployed throughout the assigned territory, with there typically being one base station located in each cell which services the mobile subscriber units traveling through that cell.
In such a scenario, only a fixed number of transmit and receive operating frequencies are thus made available to service the mobile units in each cell. One way to cope with the ever-increasing demand for cellular services is to shrink the size of each cell. However, as the density of cells increases, the number of times that a given mobile unit crosses a cell boundary also increases. This movement of a mobile unit across a cell boundary must be detected, so that the mobile can be reassigned a new pair of frequencies on which to operate in the new cell.
This process, known as hand-off, must occur quickly, so that no interruption of a call in progress can be perceived. Unfortunately, at certain cell densities, the time to process a hand-off may become a significant factor in the ability of such systems to consistently provide reliable telecommunication service.
There are at least two factors which determine the speed at which a hand-off must occur, including (1) the rate at which the mobile unit passes through the cells, and (2) the extent to which non-uniformities in the radiated electromagnetic field in the cell affect the ability to accurately detect the signal from the mobile unit. Both of these factors depend upon time required to accurately determine the relative location of the mobile unit. With respect to the speed of movement through the cells, in certain proposed PCS systems, the cells may be as small as five hundred (500) feet in radius. Thus, a mobile unit traveling only a few feet may require the handing off of the unit from one base station to a second and perhaps to even a third base station.
With respect to the second factor, because electromagnetic fields are usually non-uniform, a measurement of signal strength is typically made a number of times and then averaged. The time required to perform this measurement becomes longer as the susceptibility of the electromagnetic field to fading effects increases, such as may occur in an urban environment.
Diversity combining techniques can be used to compensate for fading by generating a number of signal transmission paths, or diversity branches, each of which carry the same information signal, but which have uncorrelated multipath fadings. The diversity branches are then combined in some way to resolve the actually transmitted signal. It would be desirable to reduce the complexity of the operations required in detecting the position of a mobile unit, by taking advantage of diversity combining techniques in as efficient a manner as possible, with a minimum number of base station antennas and associated receiver processing and control equipment.
This can, however, be especially difficult in the case where the basestation makes use of a wideband receiver which provides signals from many different remote units at the same time, because the multipath fadings must be compensated for each radio channel.